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Transmissions gone wild!

August 6, 2013

The Range Rover Evoque has been a development vehicle for the ZF 9HP nine-speed transverse automatic transmission. Photo by ZF

If you've been car shopping recently, or just paying attention to Autoweek car reviews, you've noticed that the six-speed automatic -- de rigueur as recently as a year ago -- suddenly has ratio envy. The new hotness? Eight- and nine-speed automatic transmissions.

We recently paid a visit to supplier ZF's Gray Court, S.C., plant where the company builds its 8HP and 9HP automatic transmissions; considering it left our tiny little minds blown, it occurred to us that a brief introduction to these multispeed monsters might be helpful for those of you who, like us, find even the Powerglide an engineering marvel.

Oh, and lest you fret about the complexity (and potential for future foreclosure-inducing repair/rebuild costs) of these modern eight- and nine-speed automatic transmissions, remember: They're saving you from the CVT. For that alone, we extend a hearty welcome and “thank you.”

The 8HP 8-speed automatic transmission, shown here in hybrid form with an electric motor replacing the torque converter. Photo by ZF

8HP

First up is the eight-speed automatic, which in the case of ZF's design is engineered for longitudinal applications -- in other words, the traditional RWD arrangement with the engine facing north/south and the transmission located between the two front seats.

While this layout is standard for rear-drivers, with the modern world and its AWD slap-happiness, ZF also offers a version of the 8HP with a gearset that sends power through a shaft to the front wheels; it's always used in conjunction with rear-drive, so there are no FWD-only 8HP applications.

For hybrid applications, there's a version of the 8HP that replaces the traditional torque converter with an electric motor. The electric mechanism deals with all the power transfer and adds additional oomph when necessary. There's also the ability to add a stop/start module in conjunction with or separate from hybrid capability.

Note that we said “module.” That's one of the cool things about the 8HP: Various functions are packaged in modular form, so automakers can spec an 8HP for their specific needs, adding AWD, hybrid and stop/start capability as needed. With stop/start enabled, ZF claims up to an 11 percent improvement in fuel economy over its own six-speed automatic.

Automakers using the ZF 8HP include Audi, BMW, Chrysler and Jaguar/Land Rover.

Not only does the 9HP add an extra ratio for a total of nine speeds, but it's also engineered for transverse applications -- the typical front-wheel-drive vehicle with the engine aligned east/west. ZF has managed to shoehorn all those extra ratios into approximately the same case space as a modern six-speed automatic by nesting different elements inside one another and using interlocking “dog clutches.”

Dog clutches connect via mechanical interference rather than using friction; the result is efficient power transfer, but smoothness isn't their strong suit. Getting the shift quality right was one of the big challenges for ZF engineers.

Like the 8HP, the 9HP uses a modular design approach that allows AWD and stop/start capability; ZF claims as much as a 16 percent combined fuel-economy increase compared to equivalent six-speed transverse automatics.

Manufacturers using the 9HP include Chrysler and Jaguar/Land Rover.

While ZF is the only manufacturer building eight-plus-speed automatic transmissions at the moment, others are nipping at the company's heels. GM and Ford have announced a joint venture to design nine- and 10-speed automatics, while Volkswagen is reportedly working on a 10-speed dual-clutch automatic.

Where will it end? Technically, the sky's the limit, but ZF boss Stefan Sommer says nine speeds may be where his company stops.

“There is no hard line, but you have to consider the law of diminishing returns. The question is whether adding even more gears makes sense.”

Andrew Stoy
- Digital editor Andrew Stoy is an avid enthusiast of all cars who has spent the past 15 years writing about the automobile in print, online and for advertising and PR firms.
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